1. Technical Field
The present invention relates to a cathode ray tube (CRT) display apparatus with reduced stray magnetic fields.
2. Prior Art
The picture on a CRT display device, such as computer visual display unit or a television receiver, is typically produced by scanning an electron across the CRT screen in a raster pattern. The picture is then sequentially generated by amplitude modulating the electron beam with an input video signal as the electron beam is scanned across the screen.
The raster scan is produced by deflecting the electron beam with time-varying magnetic fields produced within the CRT. The magnetic fields are produced in the CRT by energising line and frame scan coils mounted in a yoke around the neck of the CRT with line and frame scan currents each having a sawtooth waveform. The time-varying magnetic fields also extend beyond the confines of the CRT and the yoke and may radiate from the display device as stray magnetic fields. The stray fields can produce an unwanted deflection in neighbouring display devices. The unwanted deflections can produce undesirable visual disturbances in pictures displayed on the neighbouring devices. The stray field from the frame scan coil tends to produce more objectionable disturbances than that from the line scan coil. If the unwanted deflection is exactly the same frequency and phase as the normal deflection, then there is no visible interference. However, this is usually not the case. The following two arrangements are more typical. In the first arrangement, adjacent CRT displays are displaying pictures based on video signals in the same video format, but from different system units. Thus, the line and frame scan frequencies received by the displays are slightly different and are not phase-locked. The resulting visual disturbance is a dark (or light depending on the display orientation) bar slowly rolling up or down the screen. The bar is the result of the fast field change of the vertical retrace of the interfering display. In the second arrangement, adjacent displays receive video signals in different video formats, so that the related scan frequencies are different. Vertical retrace interference occurs in the same way as mentioned above in connection with the first arrangement, but this time at random phases so that the interference is seen as an undesirable flicker effect.
The above mentioned problems caused by placing CRT displays next to each have been solved by enclosing at least the electron guns and the yokes of the CRTs in high magnetic permeability, ferro-magnetic shields. Examples of such solutions are described in U.S. Pat. No. 4,864,192 and EP-A-0 348 205. The shields prevent stray fields generated in adjacent display devices from entering the CRT. Conventionally, the shields are formed from Mumetal (trade mark of Telcon Metals Limited, Crawley, Sussex, England). However, Mumetal shields are relatively expensive, thereby mitigating against volume manufacture.
Another conventional solution is to mount cancellation coils on the yoke of the CRT in a CRT display device. The cancellation coils are connected in series with the frame scan coils. In operation, the sawtooth scan current flowing in the frame scan coils also flows in the cancellation coils. The cancellation coils are oriented to generate, as a function of the sawtooth current flow, a magnetic cancellation field in anti-phase with the stray field from the yoke. The cancellation field destructively interferes with the stray field to reduce the net magnetic field strength of the stray field. Thus, the interference effect on any adjacent display is reduced. Such coils do not prevent external field sources from affecting the display device to which they are fitted. Therefore, to be effective, all displays in an installation should be fitted with cancellation coils. The problem with this arrangement is that the cancellation coils complicate the construction of the yoke assembly resulting in a more expensive end product. Furthermore, the space occupied by the coils may limit freedom in the mechanical design of the display or in the placement of components on the display drive circuit boards.
GB 2 217 959 A describes a lower cost solution in which a pair of cancellation coils connected in series with the frame scan coils of a CRT display device. The cancellation coils are placed either at the sides of the CRT or above and below it. The dimensions of the cancellation coils are commensurate with those of the CRT. Once again, the scan current flowing through the frame scan coils also flows through the cancellation coils to generate a cancellation magnetic field in anti-phase to the stray field from the frame scan coils. The cancellation field destructively interferes with the stray field to reduce the net magnetic field strength of the stray field, thereby reducing the effect of the stray field on adjacent displays. This solution, however, does not provide a sufficient reduction in the stray fields to prevent visual interference when two or more displays are packed tightly into the same installation.